Our long range objective is to determine the three-dimensional structures of biologically and medically relevant proteins of the vascular system. The relationships between structure and function will be examined. X-ray crystallographic techniques are being applied to the hemoglobin-haptoglobin complex. Crystals have been obtained and unit cell parameters determined. Heavy atom derivatives will be prepared using our knowledge of hemoglobin chemistry. Electron density maps at low resolution will be calculated showing the structure of the complex, interaction sites of the two proteins and any gross modifications in the conformation of the hemoglobin molecule. The hemoglobin-haptoglobin complex is also being examined biochemically. Using fluorescence quenching due to the heme, the binding of modified hemoglobins and haptoglobins has been followed identifying a number of the residues involved in the contact. A photoactivatable reagent has been used to crosslink hemoglobin to haptoglobin and the sites will be identified. Disassembly and reassembly of haptoglobin and their influence on hemoglobin is also being examined. The possible contact between these two proteins is being analyzed by model building using protein surface descriptions. The myeloma protein IgG2Gar binds 2 moles of riboflavin per mole antibody molecules. Attempts are being made to crystallize this protein and the riboflavin bound Fab fragment.